With a rapid proliferation of wireless devices typified by mobile phones, there has been an increasing demand for compact and high-performance filters or duplexers. The filter or the duplexer is configured by combining resonators. Hitherto, surface acoustic waves (SAWs) have been mainly used for the resonator. Recently, however, piezoelectric thin-film resonators, which are low in loss and improved in electric power resistance, electro-static destruction (ESD) property, etc., are being increasingly used.
The piezoelectric thin-film resonators are categorized into a film bulk acoustic resonator (FBAR) type and a solidly mounted resonator (SMR) type. The FBAR has a configuration wherein, as main components, an upper electrode, a piezoelectric film, and a lower electrode are arranged on a substrate, and a cavity is provided below the lower electrode in a region where the upper electrode and the lower electrode oppose each other. The cavities are classified, for example, into the following three kinds from the view point of configuration. The first is a cavity formed into a shape penetrating from the surface of the substrate to the back surface thereof. The second is a cavity formed into a shape having a depression in the surface of substrate. The third is a cavity arranged in air bridge manner on the substrate surface.
The SMR has a structure wherein, instead of using the above-described cavity, films having a high acoustic impedance and films having a low acoustic impedance are alternately laminated into a laminate film that has a thickness of λ/4 (λ: a wavelength of an acoustic wave) and that is utilized as an acoustic reflecting film.
In the FBAR or the SMR, upon applying a high-frequency electric signal between the upper electrode and the lower electrode, an acoustic wave is excited under an inverse piezoelectric effect, in a region (membrane region) wherein the upper electrode and the lower electrode oppose each other with the piezoelectric film therebetween. On the other hand, under a piezoelectric effect, a distortion due to the acoustic wave is converted into an electric signal. The acoustic wave is reflected on side end faces of each of the upper electrode and the lower electrode, thereby constituting thickness extensional vibration waves each having a main displacement in the thickness direction. In this structure, resonance occurs at a frequency at which the total film thickness H in the membrane region becomes an integral multiple (n-th multiple) of a ½ wavelength of the acoustic wave. Letting the propagation speed of an acoustic wave, determined by a material be V, the resonant frequency F is given by F=nV/2H. By taking advantage of this resonance phenomenon to control the resonant frequency by the film thickness, it is possible to produce a piezoelectric thin-film resonator having a desired frequency characteristic.
As a conventional art that has achieved an enhancement of the Q value of the piezoelectric thin-film resonator, especially of the anti-resonance Q value thereof, there has been known equipment wherein the outer peripheral portion of a piezoelectric film is disposed on the inner side further than the outer periphery of the region where the lower electrode and the upper electrode oppose each other (refer to, for example, Japanese Laid-open Patent Publication No. 2007-300430). With this arrangement, lateral leakage of acoustic waves is suppressed, leading to an enhancement of the anti-resonance Q value.